The present invention relates to a method for decontamination of a liquid effluent including one or more radioactive elements to be eliminated (such as strontium, ruthenium, caesium, the α emitters, such as americium, plutonium and uranium), by treatment in a fluidised bed, for example by coprecipitation.
The decontamination treatment of liquid effluents, notably radioactive liquid effluents, by coprecipitation was developed in the 1960s. It consists of introducing into the liquid element to be decontaminated preformed solid particles and/or precursor reagents of the said particles, where the said reagents react in situ in the liquid element to be decontaminated to form the said particles. These particles are chosen for their capacity to capture and retain selectively the said chemical element or elements to be eliminated. Among the solid particles capable of capturing and retaining metal elements, one may cite:                barium sulphate particles, able to capture and retain strontium;        iron and copper hydroxide particles, able to capture and retain ruthenium and the a emitters, such as americium, plutonium and uranium;        nickel and cobalt ferrocyanide particles, able to capture and selectively retain caesium.        
The decontamination treatment can be accomplished in two modes:                a discontinuous mode, in which only a predetermined volume of liquid effluent to be decontaminated introduced into a reactor is treated, in which the solid particles able to capture and retain the said chemical element or elements to be eliminated, and/or the precursor reagents of the said particles, are introduced;        a continuous mode, in which the effluent to be decontaminated, the solid particles able to capture and retain the said chemical elements to be eliminated and/or the precursor reagents of the said particles are introduced into a reactor in a continuous manner, at a constant or variable rate of flow, where the addition of the particles and/or reagents can be accomplished in a cascade of reactors.        
Whether with the discontinuous or continuous mode, it is obtained on conclusion of the treatment in the reactor, a suspension of solid particles, having captured the chemical elements to be eliminated initially present in the liquid effluent. The final outcome of the treatment consists, after this, in undertaking a step of liquid/solid separation, generally in a settling tank. This step can be facilitated by adding a coagulating agent and/or a flocculating agent to the suspension. The solid phase obtained at the outcome of this separation step (called at this stage a “sludge”), is then considered as a final waste product, and is conditioned, generally in bitumen or in a cement matrix, before being stored. The decontaminated liquid, for its part, is discharged into the environment, if its radiological and chemical composition permits this. Failing this, the liquid can be subject to another decontamination process.
To implement such a decontamination method two devices are traditionally required:                a reactor (which may include one or more tanks), in which the step of bringing the solid particles able to precipitate into contact with the chemical elements to be eliminated from the liquid effluent is accomplished; and        a settling tank and/or a filtration module allowing the solid-liquid separation, i.e. between the phase including the precipitate having captured the chemical element(s) to be eliminated and the liquid phase including the eliminated effluent or, at least, which has had its content of the chemical element(s) to be eliminated reduced, where the settling requires the use of organic compounds, such as flocculating agents, in order to facilitate the consolidation of the particles in the suspension.        
This type of treatment therefore requires an installation of substantial size, and which is also therefore inflexible. Moreover, the investment required for the construction, operation and maintenance of this type of installation is substantial.
Furthermore, notably when the treatment is undertaken in discontinuous mode, in order that the various steps of the treatment are able to be undertaken (filling of the tank, addition of the reagents, mixing, draining, etc.), the time required to undertake them is substantial (of the order, for example, of several hours), thereby restricting the installation's treatment capacity.
When the treatment is accomplished in continuous mode, in order to have an effective treatment, it is necessary that the effluent is able to stay for a substantial time in the reactor. Indeed, the longer the contact time between the solid particles and the chemical element(s) contained in the effluent, the greater the transfer of the elements from the liquid phase to the solid phase (the staying time must be at least 5 minutes in the reactor and at least 30 minutes in the settling tank in order to obtain satisfactory separation). Therefore, for a given effluent rate of flow, the volumes of the reactor and the settling tank must be adjusted in order to satisfy these staying times, bearing in mind that the greater the desired decontamination efficiency the larger must be the size of the reactor.
Moreover, a second treatment is sometimes required with the installations of the prior art, notably for complex effluents (containing several chemical elements to be eliminated), or those with high radiological activity. The consequence of this is to increase substantially, in the case of such effluents, the quantity of sludge produced. In order to prevent this it would be advantageous to be able to increase the treatment efficiency without increasing the volume of sludge produced.
One of the factors determining the efficiency of the treatment in terms of decontamination is the quantity of solid particles present in contact with the effluent to be decontaminated. To ensure a minimum degree of efficiency for the treatment, substantial quantities of reagents must be used. Since the radiological activity of a suspension may result from the presence of an infinitesimal quantity of radioelements (for example, several nanograms per liter), the sludge resulting from the treatment may then be weakly radioactive. It would, thus, be particularly advantageous to be able to concentrate the radiological activity in a smaller volume of sludge, in order to reduce the storage volumes.
Thus, there is a real requirement for a method of decontamination of a liquid element of one or more radioactive elements contained in it, enabling the following disadvantages to be overcome:                the problem of encumbrance of the installation on the ground, due to the need for the two operations required for the decontamination (treatment+solid-liquid separation);        the risks of contamination when the treated effluent and of the sludges formed in the solid-liquid separation unit are transferred, since this transfer occurs by means of pipes connecting the treatment unit to the decontamination unit;        the use of a large quantity of solids for a given volume of treated effluent for a decontamination which is sometimes low, and which leads to a large volume of waste, which must then be conditioned;        the need to be able to use organic compounds, such as flocculating agents, in order to accomplish the solid/liquid separation.        